The present invention is a process for making poly(ethylene-co-isosorbide) terephthalate polymer that has low color and low diethylene glycol (DEG) content.
The diol 1,4:3,6-dianhydro-D-sorbitol, referred to herein as isosorbide, is readily made from renewable resources, such as sugars and starches. For example, isosorbide can be made from D-glucose by hydrogenation followed by acid-catalyzed dehydration.
Poly(ethylene-co-isosorbide) terephthalate polymer (PEIT) is a polymer with a higher glass transition temperature (Tg) than polyethylene terephthalate (PET). This positions it for use in products such as bottles, hot-fill containers, film, thick sheet, fibers, strand and optical articles. In many of these markets, aesthetics are important, and having a very low color resin is highly desirable. To maximize the Tg-enhancing effects of isosorbide, it may also be desirable to minimize the presence of Tg-reducing impurities, such as diethylene glycol (DEG).
U.S. Pat. No. 5,912,307 discloses the use of tetramethylammonium hydroxide (TMAH) to suppress DEG formation in the melt polymerization of ethylene glycol with mixtures of the aromatic diacids, terephthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylic acid.
U.S. Pat. No. 5,959,066 discloses polyesters that include isosorbide as a co-monomer and methods for making them (Charbonneau et al.). This patent discloses process steps to obtain isosorbide-containing polymers from esterification to solid-state polymerization. No mention is made concerning the need to obtain polymer with low color. Example 2 describes the preparation of isosorbide-containing polyesters with 1% DEG content without the use of TMAH.
U.S. Pat. No. 6,063,465 discloses the range of isosorbide content in PEIT resins suitable for making polyester containers, processes for making such resin, and a method for making containers from that resin. Melt polymerization processes are described using either dimethyl terephthalate or terephthalic acid as the acid component. This patent is hereby incorporated by reference (Charbonneau, Johnson).
U.S. Pat. No. 6,063,464 describes isosorbide-containing polyesters and methods for making same (Charbonneau et al.). The patent claims the composition of isosorbide containing polyesters and a process for their solid-state polymerization. The possible applications include beverage bottle, film or sheet, fibers, optical materials, and compact disc or digital versatile disc. The patent does not make mention of any process conditions aimed at the minimization of color or DEG content.
U.S. Pat. No. 5,958,581 describes polyester film and methods for making the same (Khanarian et al.). Film comprised of isosorbide-containing polyester is claimed. Several compositions are claimed. No mention is made concerning color or DEG content.
U.S. Pat. No. 6,063,495 describes PEIT fiber and methods for making the same. It is noted that DEG may be produced as a by-product of the polymerization process. It is noted that polymer without DEG is obtainable in a solution polymerization process. However, there is no disclosure concerning minimizing DEG formation in a melt polymerization process. There is also no mention of color formation.
This invention provides a melt polymerization process for the preparation of poly(ethylene-co-isosorbide) terephthalate (PEIT), comprising:
a) providing a mixture comprising terephthalic acid or its alkyl ester, ethylene glycol and isosorbide, wherein the molar ratio of diols to terephthalic acid or its alkyl ester is from about 1.05:1 to about 1.3:1 and the molar ratio of ethylene glycol to isosorbide is from about 1.2:1 to about 24:1;
b) reacting the mixture in an inert atmosphere at a temperature in the range 180-255xc2x0 C. and a pressure in the range of 0-60 psig, with concurrent removal of a distillate comprising water or volatile alkanol products derived from the reaction of terephthalic acid or its ester with ethylene glycol and isosorbide, wherein the distillate contains less than about 5 wt % ethylene glycol and less than about 1 wt % isosorbide; and
c) continuing the reaction in the presence of a polycondensation catalyst at a pressure of about 0.25 to about 2 mm and a temperature of 260 to 275xc2x0 C. to form a PEIT having a Hunter b* color value between about xe2x88x922.0 and about +2.0.
The present invention also relates to the low color PEIT polymers made by this process.
The present invention also relates to bottles, hot-fill containers, films, thick sheet, optical articles, fibers, strand and polymer blends and alloys made from the PEIT polymer of the process described herein.
The present invention is a process to make PEIT polymer that has low color for use in hot-fill container, bottle, thick sheet, film, fiber, strand, optical articles and other applications. Color is commonly expressed in terms of Hunter numbers which correspond to the lightness or darkness (xe2x80x9cLxe2x80x9d) of a sample, the color value (xe2x80x9ca*xe2x80x9d) on a red-green scale, and the color value (xe2x80x9cb*xe2x80x9d) on a yellow-blue scale. It is usually desired to produce polymers with xe2x80x9cLxe2x80x9d between 80 and 100, preferably 90 to 100. Similarly, for low color polymers, xe2x80x9ca*xe2x80x9d and xe2x80x9cb*xe2x80x9d are preferably between about xe2x88x922.0 and about +2.0, more preferably between about xe2x88x921.0 and about +1.0, as measured by the process described herein. It has been found that these objectives can be met for PEIT without the use of color-correcting additives by controlling critical process parameters at each stage of the process, especially temperature and pressure.
For low color PEIT, it is also important to eliminate, or at least minimize, the color-forming impurities present in the monomer diols. Preferably, the UV absorbance of ethylene glycol and isosorbide are less than 0.20 at 220 nm, more preferably less than 0.10.
The choice of polycondensation catalyst also influences the color of the final polymer. Suitable catalysts include Sb(III) or Ti(IV) salts; acetate and other alkanoate salts of Co(II), and Sb(III); oxides of Sb(III); oxides of Sb(III) and Ge(IV); and Ti(OR)4, where R is an alkyl group having 2 to 12 carbon atoms. Glycol solubilized oxides of these metal salts may also be used. Oxides of Ge such as GeO2 are preferred. The preferred amount of polycondensation catalyst is generally from about 10 to about 300 ppm by weight. More specifically, the molar ratio of catalyst to terephthalic acid or its ester is about 1:1000 to about 1:7300, preferably about 1:2200 to about 1:4400.
Inclusion of the isosorbide monomer raises the Tg of the final PEIT polymer (relative to PET), while DEG incorporation into the polymer tends to lower the Tg. DEG can be formed, and subsequently incorporated into the polymer, when terephthalic acid is used in the polymerization process. For applications in which low DEG is desired and the polymerization process uses terephthalic acid, one can add a suitable base and also minimize the ratio of ethylene glycol to diacid in the initial esterification reaction. It has been found that adding suitable bases with the monomers charged to the reactor suppresses DEG formation in PEIT polymers. Suitable bases include sodium acetate, sodium hydroxide, and tetramethylammonium hydroxide (TMAH). An effective amount of base is about 10 to about 300 ppm, based on terephthalic acid. In examples herein, the combination of germanium oxide (GeO2) and TMAH give especially low color and low DEG content polymer when terephthalic acid is used in the process. For applications needing high Tg values, the DEG content is preferably less than about 1.5 mol %, more preferably less than about 1.0 mol %.
The polymerization process of this invention is a condensation polymerization of ethylene glycol, isosorbide, and terephthalic acid or its alkyl ester. Suitable terephthalic acid esters for the process of this invention include mono- and di-alkyl esters of terephthalic acid, wherein the alkyl group is chosen from the group of C1 to C6 alkyls. Dimethyl terephthalate is a preferred terephthalic acid ester. The molar ratio of diols (ethylene glycol and isosorbide) to terephthalic acid (or its ester) is from about 1.05:1 to about 1.3:1, and the molar ratio of ethylene glycol to isosorbide is from about 1.2:1 to about 24:1, preferably from about 6:1 to 18:1.
The polymerization process can be carried out in either batch, semi-continuous or continuous mode. The process is best carried out in a reactor equipped with a distillation column and a stirrer or other means for agitation. The distillation column separates the volatile product of reaction (water and/or alkanol) from volatile reactants (e.g., ethylene glycol and isosorbide). Use of a distillation column allows for operation at a lower molar ratio of ethylene glycol to terephthalic acid, which serves to suppress the formation of DEG. When terephthalic acid is used in the polymerization process, the volatile reaction product will be water; when an ester such as dimethyl terephthalate is used, the volatile reaction product will be the corresponding alkanol (such as methanol), together with smaller amounts of water.
The reactants (terephthalic acid or its ester, ethylene glycol and isosorbide) and other optional catalysts and additives are loaded into the reactor, and if necessary, the reactor is purged to remove traces of oxygen. Inert gases such as nitrogen can be used for this purpose. Polymerization starts by heating the reactants in an inert atmosphere at a pressure between about 0 and about 60 psig and removing the water and/or alkanol and other volatile by-products via distillation. The temperature is initially increased to about 220xc2x0 C. when terephthalic acid is used or to about 180xc2x0 C. when a terephthalic acid ester is used, and then more slowly to a final temperature of between 230 to 255xc2x0 C. The bulk of the water and/or alkanol are removed over about a 1 to 8 hour period.
The pressure chosen depends on the efficiency of the distillation column and on the ratio of ethylene glycol to isosorbide. At a ratio of 9 or less, one may generally operate at about 0 psig. At higher ratios, the pressure is increased to make it easier to separate ethylene glycol from the water and/or alkanol formed during the reaction of ethylene glycol, isosorbide and terephthalic acid or its ester. However, increasing the pressure also makes it more difficult to remove volatile color-forming impurities by distillation, so it is generally preferred to operate at the minimum pressure necessary to maintain acceptably low ethylene glycol losses in the distillate.
It should also be noted that the boiling point of the reaction mixture is a function of the composition of the mixture, and more specifically of the ratio of isosorbide to ethylene glycol. At high ratios, the boiling point increases, and the higher temperature of the reaction mixture results in an increased rate of reaction and associated water and/or alkanol production. Conversely, when the ratio of isosorbide to ethylene glycol is low, the boiling point of the reaction mixture is lower, leading to a lower temperature of the reaction mixture. The overall effect of a low isosorbide to ethylene glycol ratio is that the esterification reaction proceeds more slowly and the percentage of ethylene glycol in the distillate increases.
At least 80%, preferably at least 90%, of the water and/or alkanol of reaction is removed as the temperature of the reaction mixture is increased, for example from 220xc2x0 C. (for terephthalic acid) or 180xc2x0 C. (for terephthalic acid esters), to a temperature between 230 and 255xc2x0 C. Limiting the maximum reaction mixture temperature to 255xc2x0 C. minimizes the formation of color-forming by-products. It is preferred that this step also be conducted under temperature and pressure conditions that selectively remove water and/or alkanol and return ethylene glycol to the reactor. Preferably, the distillate contains less than about 5 wt % ethylene glycol and less than about 1 wt % isosorbide. This can be achieved by adjusting the temperature of the reaction mixture so that the temperature of the vapor at the top of the distillation column (overhead vapor) does not exceed the boiling point of water (if terephthalic acid is used in the process) or alkanol (if a terephthalic acid ester is used in the process) at the reactor pressure. If the temperature of the overhead vapor exceeds the boiling point of water (or alkanol), then the temperature of the reaction mixture is lowered and no distillate is taken off until the overhead vapor temperature goes below the boiling point of water (or alkanol) at the reactor pressure.
When the temperature of the reaction mixture reaches a temperature between 230 and 255xc2x0 C. and the overhead vapor temperature drops to about 2 to 20xc2x0 C., preferably about 5xc2x0 C., below the boiling point of water or alkanol at the reactor pressure, the reactor pressure is reduced to about atmospheric pressure at a rate of about 0.5 to 5 psi/min., preferably about 1-2 psi/min. As the reactor pressure drops, additional water or alkanol will distill from the reactor. The optimal rate of pressure reduction is determined by the temperature of the overhead vapor. If the overhead vapor temperature exceeds that of the boiling point of water or alkanol at the reactor pressure, the rate of pressure reduction is decreased. Conversely, if the temperature of the overhead vapor is below the temperature of the boiling point of water or alkanol at the reactor pressure, the rate of pressure reduction is increased. If the total amount of water or alkanol removed when the reactor is at atmospheric pressure is less than the desired amount, the pressure can be lowered to about 80 mm Hg (for terephthalic acid) or to about 125 mm Hg (for dimethyl terephthalate) to further drive the esterification reaction. For other terephthalic acid esters, the pressure can be lowered to that at which the alkanol boils at ambient temperature. Generally, it is preferable to remove a total of at least 90% of the volatile reaction products (water and/or alkanol) before going on to the next stage of the polymerization process.
The next stage of the polymerization process is polycondensation, in which the esters and oligomers are reacted to form polymer, with removal of residual ethylene glycol, isosorbide and water and/or alkanol. If a polycondensation catalyst was not added with the monomers, it is added at this stage, optionally with other desired additives such as infrared absorbing agents, dyes, pigments, UV stabilizers and other thermally stable additives.
Color-correcting additives can be added selected from the group consisting of red, orange, yellow, blue, green, indigo and violet. Examples of such dyes or pigments include cobalt acetate, HS-325 Sandoplast(copyright) Red BB, HS-510 Sandoplast(copyright) Blue 2B, Polysynthren(copyright) Blue R, and Clariant(copyright) RSB violet are especially useful to lower the b* value of the PEIT polymer. The reactor pressure is then reduced to about 0.25 to 2 mm Hg, preferably to about 0.25-1 mm Hg. The temperature of the reaction mixture is raised to 260 to 275xc2x0 C. while the pressure is lowered. The reaction mixture is held at this temperature and pressure for about 1 to 4 hours to form the desired PEIT polymer. Minimizing time at high temperatures helps to minimize color generation in the PEIT polymer.
The polymer can be removed from the reactor and isolated in any of several conventional processes as strands, pellets or flake. An inherent viscosity (IV) of 0.5 dL/g or higher can be achieved by this melt polymerization process. The IV can be further increased by solid state polymerization of the isolated polymer.
The defined process conditions give a PEIT polymer product that has low color and low DEG content resin for use in hot-fill container, bottle, fiber, optical articles, film and thick sheet applications. The PEIT of this invention can also be used in making polymer blends and alloys